1. Field of the Invention
The invention generally relates to capacitors and more specifically to capacitor arrays within an analog integrated circuit.
2. Prior Art
Capacitors are well known two terminal devices. A capacitor is a passive electrical device typically comprised of conductors separated by one or more dielectric materials. The capacitor facilitates the storage of charge when a potential difference exists between adjacent conductors. The capacitance C of a capacitor is equal to Q/V, where Q is the charge stored by the capacitor and V is the voltage between the conductors.
Various techniques and materials used in manufacturing capacitors are selected to provide the desired capacitance values and other parameters associated with a capacitor, such as the maximum voltage rating, a power rating, stability with time and temperature and/or an upper and lower temperature tolerance. The capacitance C of a capacitor is dependent on the material and dimensions of the capacitor structure, namely C=μA/d, where μ is the dielectric constant of the material used for the dielectric between the conductive capacitor plates, A is the overlapping area of adjacent capacitor plates and d is the separation of the overlapping plates. To increase the capacitance of a capacitor, one must increase the capacitor plate area, decrease the separation between capacitor plates and/or use a dielectric with a higher dielectric constant.
Capacitors connected in parallel increase the amount of total capacitance, while capacitors coupled in series decrease the total capacitance across the series combination. When coupled in parallel together, the capacitance value of each is summed together to obtain the overall capacitance. C1+C2=Ctotal, for example. When coupled in series together, the inverse capacitance value of each is summed together to obtain the inverse of the overall capacitance. (1/C1+1/C2)=1/Ctotal, for example.
A capacitor may be a discrete device with electrical leads that can be electrically coupled to a printed circuit board. Alternatively, a capacitor may be an integrated device a part of an integrated circuit, which may include other devices as a part of one semiconductor die.
Within the same semiconductor die or integrated circuit, a plurality of capacitors may be created using different material layers as the capacitor plates. Another layer of material may be used as a dielectric between the capacitor plates to electrically isolate the plates and increase the capacitance of the capacitor. The capacitors for an integrated circuit are typically defined by laying out manufacturing mask layers in order to form the appropriate material layers during manufacturing. The mask layers, when overlaid on top of one another, are oftentimes referred to as a layout.
A capacitor may be intentionally designed into an integrated circuit using the layout. Other capacitors arise unintentionally, as a consequence of manufacturing an integrated circuit. These unintentional capacitors are oftentimes referred to as parasitic capacitors. For example, around a metal oxide semiconductor field effect transistor (MOSFET), there may be a gate to drain parasitic capacitance and a gate to source parasitic capacitance due to how the MOSFET is manufactured.